1. Field of the Invention
The present invention relates to an image forming method used for a copier, a printer or a facsimile apparatus, and to an image forming apparatus. More particularly, the invention relates to an image forming method and an image forming apparatus using an electrophotographic method in which an image is obtained by developing an electrostatic latent image formed on an image bearing member using toner.
2. Description of the Related Art
Conventionally, an ordinary image forming method utilizing an electrophotographic method includes a charging process for charging an image bearing member, a latent-image forming process for forming an electrostatic latent image on the charged image bearing member, a developing process for forming a toner image by developing the electrostatic latent image, and a transfer process for transferring the toner image onto a transfer material.
However, in conventional image forming apparatuses utilizing the above-described image forming method, since a corona charger is used as a charging device for charging a photosensitive member, ozone is generated due to corona discharge during charging.
Recently, in accordance with increasing consciousness on environment, a contact charging method has been adopted as a charging method not using corona discharge which is accompanied with the generation of ozone. A magnetic-brush-type charging device, such as one described in Japanese Patent Laid-Open Application (Kokai) No. 61-57958 (1986), is preferably used as a charging member of the contact charging method from the viewpoint of stability in contact charging.
However, when image forming operations are repeated by using the above-described magnetic-brush-type contact charging device (magnetic-brush charger) and performing two-component (i.e., toner and magnetic carrier particles) contact development, fog is gradually produced in the formed image, and the dispersion of toner particles from the developing device increases.
The inventors of the present invention have performed various studies about the above-described phenomena of the generation of fog and the increasing dispersion of toner, and have determined that these phenomena occur due to gradual mixture of conductive magnetic particles for charging within the developing receptacle during the progress of image forming operations. The mechanism of the mixture of conductive magnetic particles for charging within the developing receptacle, and the reason of the generation of fog and the dispersion of toner particles will now be described.
In the magnetic brush charger, conductive magnetic particles for charging are carried on a charging sleeve incorporating a magnet by a magnetic force of constraint, to form a magnetic brush. By sliding of the magnetic brush in tight contact with the surface of the photosensitive member due to the rotation of the sleeve or the magnet, the surface of the photosensitive member is charged. In order to improve the charging property, the magnetic-brush-type contact charging device carries a considerably larger amount (at least twice) of magnetic particles than the amount of the developer carried on the developing sleeve of the two-component-type developing method. Accordingly, the distal end portion of the magnetic-particle layer on the charging sleeve is hardly magnetically constrained. Particularly when using a small-diameter sleeve (or magnet) and rotating the sleeve at a high speed, magnetic particles at the distal end of the magnetic-particle layer leave. The lost magnetic particles adhere onto the photosensitive member little by little, and are collected and accumulated into the developing receptacle at the developing portion in accordance with the rotation of the photosensitive member.
In image forming apparatuses in which a cleaning device is provided at a side upstream from the magnetic-brush charger in the direction of rotation of the photosensitive member, in some cases, toner particles and other additional agents pass between the cleaning blade of the cleaning device and the photosensitive member and are mixed, little by little, in the magnetic-brush charger in accordance with repeated image forming operations. As a result, the resistance of the magnetic brush increases, thereby causing a difference between the applied voltage and the potential on the surface of the photosensitive member, and adherence of magnetic particles onto the photosensitive member due to the potential difference.
In a system adopting a cleaner-less process in which a cleaning device is not provided at a side upstream from the magnetic-brush charger in the direction of rotation of the photosensitive member, a small amount of toner particles remaining on the photosensitive member after image transfer is first received in magnetic particles for charging of the magnetic-brush charger, the magnetic particles are discharged onto the photosensitive member after being stirred, and the toner particles discharged onto the photosensitive member are collected into the developing receptacle, since toner particles tend to be accumulated in the magnetic particles for charging, the above-described adherence of magnetic particles due to the potential difference tends to more easily occur.
The conductive magnetic particles for charging thus accumulated within the developing receptacle are mixed and stirred with toner particles within the developing receptacle and newly replenished toner particles. Conventionally, the relationship between the amount of charging of toner particles due to friction with the conductive magnetic particles for charging and the amount of charging of toner particles due to friction with magnetic carrier particles for development has not sufficiently been considered. For example, when the amount of charging of toner particles due to friction with the conductive magnetic particles for charging is less than the amount of charging of toner particles due to friction with the magnetic carrier particles for development, or the two types of toner particles have opposite polarities, the number of toner particles having smaller amounts of charging increases in the distribution of the amount of charging of toner particles within the developing receptacle. Furthermore, as the charging capability of magnetic carrier particles for development decreases after being used for a long time the number of toner particles having smaller amounts of charging increases, thereby increasing fog and dispersion of toner particles.
European Patent Laid-Open Application No. 0844536 (corresponding to U.S. Pat. No. 5,994,019) has proposed that by providing the relationship among the volume resistivity of the surface layer of a latent-image bearing member, the volume resistivity of a charging member of contact charging means, the volume resistivity of additional agents of toner, and the volume resistivity of a magnetic carrier of a two-component-type developer, the charging property by injection into the latent-image bearing member during charging is improved, an electrostatic latent image is not disturbed in a developing region during development, and an image having high picture quality and high durability can be formed.
However, even in the image forming method of the above-described prior art, in order to stably maintain the developing property even after a large number of repeated uses when using a small-diameter sleeve as a charging sleeve of a magnetic-brush charger, and forming an image while charging the surface of an image bearing member by rotating the sleeve at a high speed, it is necessary to perform further improvement.